Sharing, unsharing and re-sharing is not working correctly

Bug report

Unshared credentials cannot be shared again by Alice, moreover they are (somehow) duplicating password entries on Bob's side (cf. Screenshots).

Steps to reproduce

1. Alice unshares a credential -> this disappears correctly on Bob's (the receiver) side.
   
   

2. Alice tries to share the credential again, seems to work at first glance. If Alice wants to leave the share dialog the "this will corrupt the credential" error message appears
   
   

3. On Bob's side the credential does not appear again, but suddenly an other entry is listed twice. If Bob selects the duplicate entry both entries are highlighted as selected (they only seem to be doubled in the view):
   

4. Alice cannot press the "unshare" button again:
   

5. If Alice tries to share the credential again, the share process is listed as pending, but sharing to Bob did not work.
   

Expected behaviour

If Alice presses the "unshare" button at a certain credential -> the entry disappears on Bob's side
If Alice presses "share" again -> Bob's side receives a notification for the shared credential
In Bob's vault the shared credential should appear
On both sides the share icons for the credential are update

Actual behaviour

The unshare button deletes the credential on Bob's side -> that's correct
A next share of Alice leads to an incomplete share state -> a corruption error appears on Alice's side and Bob gets duplicate entries in his vault
On Alice's side neither sharing nor unsharing is working anymore.
On Bob's side the shared credential is not listed

Configuration

Operating system:
Ubuntu 16.04 on server and client side

Browser:
Tested with Chrome Version 60.0.3112.78 (Official Build) (64-bit) and Firefox 54.0 (64-bit)

Passman version:
Version: 2.1.4

Database:
MySQL 5.7.19-0ubuntu0.16.04.1

PHP version:
PHP-FPM 7.0.18-0ubuntu0.16.04.1

cloud server: Nextcloud
Nextcloud 12

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